Ok, I got a bit impatient, as I missed this page
that explained exactly how to get 2 GPU clients working. However, the clients are the Systray ones, not the console ones that I wanted. But at least they're working. I'll play around later and see if I can get two console clients running.
Anyways, performance is a little lower than what I was expecting, but still relatively high. (I'm running XP btw) Each GPU client runs between 33% and 42% CPU load, the average being about 40%. One 4830 gets 2100 PPD and the other get 2800 PPD. I don't understand why there is such a difference in PPD. Both cards are at stock speeds.
The biggest con to running the GPU client is its high CPU usage. My SMP client is only getting 20% of the CPU. It only completed two percent of the work unit in the last nine hours.
Normally it completes one percent in less than a half hour.
That's also pretty bad in terms of power consumption because not only are both GPUs at 100% usage, the CPU is also pegged at 80%, and it's not even doing anything! I don't have a kill-a-watt so I can't say which is more efficient. (PPD/Watt) 4GHz 1.3v E8400 = ~2300 PPD. 100% 4830 + 40% CPU = 2800 PPD. The GPU + 40% CPU is probably less efficient.